Bixbyite
Manganese Iron Oxide (Mn,FE)2O3
Crystal System: Isometric
Hardness: 6.0-6.5
Density: 4.9-5.0

Bixbyite is a rare mineral that occurs as opaque, black, cubic crystals with a metallic to submetallic luster. The crystals are often modified by trapezohedrons and sometimes occur as penetration twins. It occurs in cavities in rhyolite and is associated with garnet, topaz, beryl and hematite. Bixbyite is a minor ore of manganese.

Quartz
Silicon Dioxide, SiO2
Crystal System: Hexagonal
Hardness: 7.0
Density: 2.65

Quartz is by far the most abundant of the polymorphic forms of silica and the most widespread and abundant mineral of the earth's crust. It occurs in igneous, metamorphic and sedimentary rocks as well as hydrothermal veins, metasomatic and hot spring deposits.

Topaz
Hydrous Aluminum Silicate Al2SiO4(F,OH)2
Crystal System: Orthorhombic
Hardness: 8.0
Density: 2.49

Topaz is a widely distributed mineral that is associated with granites, pegmatites, rhyolites and also occurs in greisen and pneumatolytic bodies. It occurs as prismatic crystals, often with vertical striations on the faces, and is quite variable in color. White to pale yellow color is most common, and yellow to sherry colors are most often associated with topaz. However, topaz also occurs in pale blue to green colors, with natural pink to red specimens being the rarest. Many specimens have been found that exhibit cavities of various sizes which are filled with gaseous liquids or crystals of ilmenite, hematite, and rutile. Very large pieces of topaz have been found, some weighing upwards of 600 pounds.

Topaz is used as gem material by jewelers and lapidary hobbyists. The natural color of topaz may be altered by heating it in the presence of various gases, an old process sometimes referred to as "pinking." Some topaz specimens exhibit sensitivity to light, and loose color if exposed to strong sources or for long periods of time. In gem circles, the name "topaz" is rather generically used to describe yellow stones, including quartz (citrine).

Rocks from Martin Friedlander's Collection

Index of Specimen Images

Table of Contents

bkeller@rockhounds.com 8/8/95